FIG. 1 is a diagram illustrating a handover (HO) procedure for serving-cell switching from a source cell to a target cell. Hereinafter, the source cell is interchangeably referred to as cell A 100, the target cell is interchangeably referred to as cell B 120, a base station hosting the cell A 100 is interchangeably referred to as enhanced node B (eNB) A 110, and a base station hosting the cell B is interchangeably referred to as eNB B 130.
If a terminal (interchangeable referred to as mobile terminal and user equipment (UE)) moves from one cell to another across a cell boundary, the cellular wireless communication system performs a HO procedure for switching the serving cell from a source cell to a target cell. In an exemplary 3GPP LTE network system as depicted in FIG. 1, if a UE 140 moves from the cell A 100 to the cell B 120, the eNB A 100 performs the HO of the UE 140 to the eNB B 130.
FIG. 2 is a signal flow diagram illustrating a schematic HO procedure in a long term evolution (LTE) system.
In FIG. 2, if the eNB A 110 sends the eNB B 120 a HO request message to request at step S200, the eNB B 130 determine whether it can accept the request and, if so, sends the eNB A 110 a HO request acknowledgement (ACK) message, at step S210, to accept the HO request. The eNB A sends the UE 140 a HO command message, at step S220, to instruct to switch its serving cell to the cell B 120 and sends, at step S230, the eNB B 130 a sequence number (SN) status transfer message carrying the information about downlink (DL)/uplink (UL) data for data forwarding to the eNB B 130 at step S240. Upon receipt of the HO command message, the UE 140 stops data communication with the cell A 100 and initiates a procedure for a call connection to the cell B 120. In this procedure, the UE performs a random access procedure by sending a random access channel (RACH) to the eNB B 120 at step S250, and the eNB B 130 sends a random access response (RAR) message in reply at step S260. Finally, the UE 140 sends the eNB B 120 a HO complete message to compete the HO procedure with successful serving cell switching to the cell B 120. Afterward, the UE resumes data communication via the cell B 120.